oci: Install FreeBSD-pam in the runtime image

In 16.0/15.1, the PAM modules were split from FreeBSD-runtime into a
new FreeBSD-pam package. FreeBSD-runtime does not install FreeBSD-pam,
which means if a user starts from runtime, then installs sshd, sshd will
fail to authenticate users because of missing PAM modules.

Since FreeBSD-pam is relatively small (about 230kB on amd64), and is
already part of FreeBSD-set-minimal, add it to the runtime image as
well. Users who absolutely don't want this can still build their
own images without it.

MFC after: 1 week
Reviewed by: dfr
Reported by: Michael Johnson <ahze@ahze.net>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D59194